In a product such as a steam turbine rotor that is assembled by welding a plurality of components, a so-called back bead welding, in which a welding bead is formed to be melted into a back surface-side of a welding base material, is performed to improve welding strength of the product. At this time, the welding is performed by supplying an inert gas such as argon or helium from a back surface-side and a front surface-side of a welded connection portion to replace air with the inert gas and thus, the welding bead is not in contact with oxygen. As a result, welding defects are prevented from occurring, and joint performance resulting from oxidation of the welding bead is prevented from deteriorating. The welding that is performed in this way while supplying the inert gas from the back side of the welded connection portion is referred to as a back-shielded welding.
For example, as described in PTL 1, when a steam turbine rotor has a structure in which an inspection hole is provided in a region proximate to a welded portion, the back-shielded welding can be performed by using the inspection hole. However, in a steam turbine rotor without an inspection hole, since a hollow portion on a back surface-side of welding base materials is sealed, the back-shielded welding cannot be performed as it is. For this reason, a method is adopted in which abutting edge portions of the welding base materials are abutted against each other and are temporarily fixed, a round gas supply hole is punctured through an abutting portion, a tube is inserted into the gas supply hole, welding is performed while injecting an inert gas via the gas supply hole, and finally the gas supply hole is blocked by the welding bead.
For example, in a structure to which it is difficult to apply the back-shielded welding, PTL 2 discloses a welding method in which a back receiving portion is formed in abutting edge portions of welding base materials to cover a back side of a welded connection portion, welding is performed in such a manner that the back receiving portion is melted and thus, the welding is performed in a state where the back is not shielded.
PTL 3 discloses a welding method in which even though a back is not shielded, compositions of the welding base materials are adjusted in such a manner that a welding bead does not excessively oxidize and thus, defects in a welded connection portion are prevented from occurring.